Field of the Invention:
The invention relates to a method for feeding reducing agent into an exhaust-gas mass flow in an exhaust-gas treatment device, in particular for mobile internal combustion engines. The invention also relates to a motor vehicle and a stationary installation in which the method is performed.
In order to prevent the release of nitrogen oxide compounds resulting from combustion in modern internal combustion engines, in particular in diesel engines, the SCR (selective catalytic reduction) method has become established. For that purpose, an SCR catalytic converter is used which has a coating that enables the reaction to occur at relatively low temperatures. During the selective catalytic reduction, nitrogen oxide compounds are caused to react with ammonia, in such a way that nitrogen and water are formed. In order to achieve as high a conversion rate as possible, it is desirable for the greatest possible amount of ammonia to be available for the reaction. It is, however, simultaneously necessary that as little ammonia as possible exits the SCR catalytic converter or the exhaust-gas treatment device again, because ammonia is perceptible in the form of an unpleasant smell even in small quantities. The stoichiometric metering of ammonia thus represents the desired metering quantity. The metering of ammonia into the outflowing exhaust gas in the respectively required quantity is an objective that has heretofore been unachievable or achievable only with very low accuracy despite very high outlay in terms of control technology. Therefore, storage catalytic converters are often used which store excess quantities of metered ammonia and, in the event of underdosing of ammonia, make stored ammonia available again for the reaction with the exhaust gas. It is also sought in that way to prevent very frequent operation of the metering system (injectors, pumps, etc.), in order to be able to realize the least possible wear and energy consumption thereof.
A disadvantage of a storage catalytic converter is that the storage of ammonia in a storage catalytic converter is subject to numerous boundary conditions that influence the maximum storage quantity, that is to say the fill level, of the storage catalytic converter. The maximum loading is subject, inter alia, to intense temperature dependency and to aging, in particular due to thermally induced changes of the storage coating. Furthermore, measurement sensors for the controlling of the loading of the storage catalytic converter are also affected by various disturbance variables. Altogether, they can result in erroneous control events, which reduce the utilization of the storage catalytic converter or even make such utilization impossible.